Integrated circuits (ICs) generate heat as an undesirable byproduct during their use. This heat byproduct is a significant design consideration, both in the design of the IC and in the design of products incorporating the IC. One strategy for addressing this issue is to design integrated circuits so that they will generate less heat in the first place. Another strategy for addressing this issue is to control how the IC is driven during use. A further strategy is to cool the IC.
Many different tactics have been used to cool ICs. One tactic is to convectively cool the IC by applying moving air over it. This may involve, for example, the use of heat exchangers filled with liquids or gasses to remove heat generated by the IC. Another common tactic to cool ICs has been to attach a heat sink with radiant cooling fins to the IC. In such a device, the cooling fins act to wick away the heat generated by the IC. Frequently, the finned heat sink is combined with a fan, in which case the device radiates off heat or, by acting in conjunction with the fan, convectively transfers heat.
In most implementations of the tactics described above, the ICs have been housed in mechanically designed housings called IC packaging. The packaging provides the dual functionality of physically and electrically insulating the IC, while at the same time providing easy electrical contact to the IC. Since the primary purpose of the packaging is to insulate and protect the IC, the packaging has a tendency to inhibit the release of heat energy.
Prior art tactics employed to address this issue include the incorporation of a heat spreader inside the packaging, or the inclusion of a heat slug which has a surface exposed on the outer surface of the package. FIG. 1 illustrates a cross-section of a prior art surface mounted IC package employing a heat slug. This example is a thermally enhanced plastic ball grid array (TE PBGA) package. The IC package 10 is a housing for an IC 12 with an active surface 14 and a non-active surface 16, and which is affixed via an adhesive layer 18 to a substrate 20. The active surface 14 of the IC 12 is electrically connected via wire 22 to mounting surfaces 24 on the substrate 20. These mounting surfaces 24 contain traces 26 which are typically eventually electrically connected to solder balls 28. Though not shown in FIG. 1, these solder balls 28 are the means by which the packaged IC 10 is surface mounted to provide electrical connection to an electronic circuit board (not shown).
In the IC package shown, the heat slug 30 is non-electrically mounted via mounts 32 on the substrate 20 over the IC 12, and is surrounded by the molding compound 34. The slug is typically manufactured out of copper, aluminum, or steel. In the case of the particular TE PBGA depicted, the slug is approximately 300 μm thick and is made of copper. In some prior art IC packages, the heat slug 30 remains exposed as shown in FIG. 1 so that it can transfer heat more easily out of, and away from, the IC and its packaging. Other prior art IC packages employ a heat spreader fully encased in the molding compound. The heat slug configuration provides marginal beneficial effects. However, the benefits of heat slugs 30 configured in this way have limited efficiency and effectiveness, and are problematic to manufacture.
Part of the reason for its limited effectiveness is that the heat slug is too far from the IC 12 heat source. The packages illustrated in FIG. 1 are typically rated to handle two to three watts (2 W-3 W). Additionally, the space between slug 30 and IC 12 is typically filled with molding compound 34. Because of its low thermal conductivity, the molding compound acts to thermally insulate the IC 12. This fact may be appreciated from TABLE 1 below, which compares the thermal conductivity of a typical molding compound such as G760 with the thermal conductivities of some other materials typically present in the device.
TABLE 1MaterialThermal Conductivity (W/mK)Silicon148.0Copper386.0Aluminum222.037-63 Solder50.7molding compound0.7~0.9
The lack of proximity between the slug 30 and the IC, and the thermally insulating properties of the molding compound, limit the efficiency of removing heat from the IC 12. While it may be possible to move the slug 30 closer to the IC, this creates design issues because, in such a position, the slug can interfere with the wire leads 22 to the IC 12 and the substrate circuit 26, and complicates the formation of the molding compound 34 around the IC 12 and the slug 30.
Another limitation of some prior art heat slugs is that they require the use of mounts 32 to maintain distance between the slug 30 and the IC 12. The use of mounts 32 is undesirable in that they require landing areas (not shown) on the substrate which must be accommodated when designing the substrate circuits 26.
FIG. 2 is an illustration of another prior art IC package 40. In this package, an interposer layer 42 rests between the IC 12 and the heat slug 44. Typically, in such a package, a conductive adhesive layer 46 is disposed between IC 12 and interposer layer 42, and a conductive adhesive layer 48 is disposed between the interposer layer 42 and the heat slug 44.
Although the package in FIG. 2 has better thermal properties, it is difficult to manufacture. In particular, if the accumulated vertical dimensions are on the larger side of acceptable tolerances, when the device is clamped in the mold tool, the IC may be crushed or damaged. If the dimensions are adjusted to avoid crushing of the die, the mold compound will tend to extend over the top of the spreader, thus leading to poor thermal performance. Hence, the tolerance accumulated from the thicknesses of the components and adhesive layers will cause the manufacturing process to vary between damage due to crushing and poor thermal performance or molding placement.
There is thus a need in the art for removing heat from an IC that is more efficient and/or easier to manufacture, and is therefore more effective. These and other needs are met by the devices and methodologies described herein. Based on a preliminary analysis, it is believed that the improvements to IC packaging described below may produce a doubling of the wattage capacity of the packaging.